The steam generators of such power stations frequently have a ring-shaped distributor, whose shape is like a vertically axed torus or with lobes and which is supplied by a substantially horizontal pipe. During the normal operation of the reactor, the free surface of the water in the enclosure is located above the pipe and the distributor and the water speed in the pipe is relatively high. Operation is then satisfactory. However, it can be disturbed under exceptional, temporary conditions of various types.
One problem is caused by the thermal stratification of the feed water when operating under very low loading conditions. This phenomenon is more particularly described in the article "Loading conditions in horizontal feed water pipes of LWRs influenced by thermal shock and thermal stratification effects", by M Miksch et al, NUCLEAR ENGINEERING AND DESIGN, 84, 1985, pp 179-187. This phenomenon is aggravated in the case of steam generators equipped with a single feed means for the water, which is under normal loading conditions, but in the case of an accident or problem it is under very low loading conditions. In the latter situation, a small feed water flow is injected into the pipe full of water at the operating temperature of the steam generator. As the flow rate is low, the cold water does not mix with the hot water and instead forms a separate layer at the bottom of the pipe. This stratification maintains a high temperature gradient between the upper and lower parts of the pipe wall, which leads to thermal stresses of the type liable to cause cracking, including in the weld joining the pipe to the enclosure. This phenomenon is aggravated by the fatigue caused by vibrations of the interface between the layers. Curved pipes which are supposed to assist the accumulation of cold water in a lower level upstream part where the areas to be protected are located do not significantly reduce this stratification phenomenon, which would seem to indicate that it is helped more particularly by the heat exchange resulting from the natural convection between the hot water occupying the balloon-shaped means and the interior of the pipe. Various attempts have been made to eliminate this stratification. One consists of introducing feed water into the pipe via an annular spill port issuing into a water box where a certain amount of mixing takes place It has also been proposed to provide on the inner face of a bend or elbow of the pipe, an annular or spiral projection of limited height, in order to increase the turbulence of the flow. Such a device has only very limited effectiveness. Thus, when the flow rate is low, i.e., when a mixture is sought, the stream of feed water merely passes round the obstacle thus formed.